Fehling solution B

Fehling’s Solution B is one of two separate solutions used in Fehling’s test, a classical chemical test to detect reducing sugars. It contains potassium sodium tartrate (Rochelle salt) dissolved in a strong alkaline solution of sodium hydroxide.

🏭⚗️ Production
Fehling’s solution B is prepared by dissolving potassium sodium tartrate (Rochelle salt) in an aqueous solution of sodium hydroxide (NaOH). It is typically mixed in equal volume with Fehling’s solution A (which contains copper(II) sulfate) immediately before use, as the combined solution is unstable over time.

🔬 Properties
Fehling B is a clear, colorless to slightly yellow alkaline solution. It provides the alkaline medium and complexing agent (tartrate ions) necessary to keep copper(II) ions from Fehling A in solution. When combined with Fehling A and heated in the presence of reducing sugars, a brick-red precipitate of cuprous oxide (Cu₂O) forms, indicating a positive result.

🧪 Applications
Fehling’s solution B is used in combination with Fehling A for qualitative testing of reducing sugars such as glucose, fructose, and lactose. It is used in food analysis, educational labs, and classical biochemistry to detect the presence of aldehyde functional groups in sugars.

⚠️ Safety
Fehling’s B is strongly alkaline and can cause skin and eye irritation or chemical burns. Avoid contact and handle with gloves and protective eyewear. The combined Fehling solution (A+B) should be freshly prepared and disposed of properly after use. Store Fehling B in a tightly closed container at room temperature, away from acids and incompatible substances.